Fig. 1. (A) The simplest two-dimensional passive dynamic walking model has two degrees of freedom, stance leg angle and swing leg angle, and is restricted to motion in the sagittal plane. Mass is concentrated in points located at the pelvis (M) and feet (m), making it possible to compute step-to-step transition costs analytically (Garcia et al., 1998; Kuo, 2002) (B) The anthropomorphic three-dimensional passive dynamic walking model (Kuo, 1999) extends this model in two ways. First, it employs a torsional hip spring acting between the limbs, making it possible to explore the mechanics of walking at different step lengths or frequencies (after Kuo, 2002). Second, it includes an extra degree of freedom allowing for lateral motion and finite step widths. Step width is adjusted by changing the splay angle, ß. The model has three degrees of freedom (stance, swing and roll angles).